Sterile, Endotoxin-Tested, Non-Pyrogenic: Three Different Claims

Three words appear on peptide listings as if they were synonyms: sterile, endotoxin-tested, non-pyrogenic. They describe different tests with different results, and a vial can satisfy one while failing another. This matters because the word on the listing is often the only evidence offered, and the three are not interchangeable.

What each term actually means

• Sterile means no viable organisms are present. It is a property of the whole container, achieved by a validated process — filtration into a sealed vial under controlled conditions, or terminal treatment of the sealed unit — and demonstrated by a sterility test on samples from that batch. Sterility says nothing about what dead organisms left behind.
• Endotoxin-tested means a quantitative assay was run for bacterial endotoxin, which is a heat-stable fragment of Gram-negative cell wall. The result is a number with units: endotoxin units per milligram or per millilitre. Endotoxin survives processes that kill the organism that produced it, which is exactly why the two tests exist separately.
• Non-pyrogenic is a claim about not causing a fever response. In practice it is usually inferred from an endotoxin result, but endotoxin is not the only pyrogen, so the term is broader than the test that typically backs it.

Why a sterile vial can still fail on endotoxin

Sterilisation removes the organism, not its residue. Material grown or handled in conditions with Gram-negative contamination can be filtered perfectly and sealed perfectly and still carry a high endotoxin load, because autoclaving and filtration do not destroy or remove endotoxin reliably. The reverse case also exists: a vial with a very low endotoxin figure that was never processed aseptically is not sterile. A claim on one of the two tells you nothing about the other.

What a credible claim looks like

• A number, with units. “Endotoxin: <1 EU/mg” is a result. “Endotoxin tested” with no figure is a statement that a test happened.
• A method. LAL-based assays are the normal route; which variant was used affects what the number means.
• A lot number matching the vial. Endotoxin and sterility are properties of a specific fill, not of a product line, and a result from a previous lot does not transfer.
• A date, because these are results from a moment in a process, not permanent attributes.
• For sterility, what was done — filtration plus aseptic fill, or terminal treatment — and that a sterility test was run on that batch.

Where the claims usually come apart

• The product is lyophilised in a vial that was not sealed aseptically. It can be clean and dry and still not meet any definition of sterile.
• The test was run on the bulk peptide, not the filled vial. The fill step is where contamination is introduced, so a bulk result is the less informative of the two.
• “Sterile water” is doing the work. The solvent being sterile says nothing about the vial contents it is added to.
• A single historical certificate is reused across lots. This is the most common gap and the easiest to check: compare the lot number on the report to the one on the label.

What to ask

Ask for the endotoxin figure with its units and the lot it belongs to; ask whether it was measured on the filled vial; and ask separately whether any sterility claim is being made and on what basis. A supplier that answers “we do not claim sterility, and here is the endotoxin result for this lot” has given you more usable information than one whose listing says all three words with nothing behind them. Endotoxin testing covers how the assay works and what the figures mean; quality claims covers the other words that appear on these listings.

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